Dicharax ovatus Páll-Gergely & Auffenberg n. sp.

Fig. 31A

Material examined. Holotype: Thailand, Surat Thani Province, Highway 401, 2.7 km W junction Highway 4142 and 401, evergreen forest on rocky hillside, 9°10’N, 99°40’E, 90 m a.s.l., 19 April 1988, K. Auffenberg leg., below cliff, base of cliff, UF 345330 (D: 1.86 mm, H: 0.99 mm) . Paratypes: 5 shells, same data as holotype, UF 551223 . Other Material: 3 broken shells, same data as holotype, UF 551224 .

Etymology. Named for the ovate (Latin derivation: ovatus) shell shape (in dorsal view).

Diagnosis. A tiny Dicharax species with depressed shell, finely ribbed R1, R2+R3 less than a quarter whorl, R2 with slightly curved ribs shorter than R2.

Description. Shell off-white, light yellowish; shell outline oval in dorsal view, spire depressed, nearly flat; body whorl rounded; protoconch low, rather glossy, 1.25–1.5 whorls; R1 of 1.25 whorls; R1 with fine, regular, dense ribs, gradually become more widely-spaced and elevated toward R1 termination; boundary between R1 and R2 not conspicuous but discernible due to denser R2 ribs than those of R1; R2 with low ribs that slightly curve toward aperture, ca. 24–26 R2 ribs present; R2+R3 less than quarter whorl (ca. 80˚); R2 slightly shorter than R3; R3 with low ribs of similar density to those on R1; boundary between R2 and R3 clear due to change in rib density and shallow constriction; very low swelling on R3 only indicated; aperture slightly oblique to shell axis, rounded; boundary between inner and outer peristomes distinct; inner peristome thin, slightly protruding and expanded, rather narrow, outer peristome thicker than inner peristome, strongly expanded but not reflected; umbilicus wide, sigmoid, more than a third of shell width.

Measurements. D: 1.81–1.95 mm, H: 0.93–1.1 mm.

Operculum. unknown.

Differential diagnosis. The most similar species regarding size and the dense, fine ribbing is the nearby occurring Chamalycaeus canaliculatus (examined material: Golf von Siam: Koh-Samui, coll. Möllendorff, SMF 109468, lectotype), which has a more pronounced constriction at border of R2 and R3, and a narrower swelling on R3 .

Chamalycaeus armillatus (Benson, 1856) has a more elevated spire, the first 1–1.5 whorls of the teleoconch are smooth and glossy, and the last whorl of R1 has spiral striation. Dicharax parvulus (Möllendorff, 1887) (examined material: Malakka: Bukit Pondong (Perak), coll. Möllendorff, SMF 109507, lectotype, Fig. 31B) is smaller, has a comparatively larger aperture, a more thickened peristome, and a shorter R2 .

Distribution. This new species is known only from the type locality (Fig. 3).